Video image streams are often recorded, transmitted, and displayed in two different video image formats: an interlaced format (also commonly referred to as the interlaced scan format) and a progressive format (also commonly referred to as the progressive scan format).
In the interlaced format, each video frame, which is a raster array of pixels representing an image, includes an odd field and an even field. The odd field includes pixels located on odd-numbered horizontal scan lines of the frame and the even field includes pixels located on the even numbered horizontal scan lines of the frame. Thus, each field contains approximately half the information content, i.e., pixels, of the complete video frame. When displayed on an interlaced-based display device, such as a conventional television, video images are presented in the form of a rapid sequence of fields, in which odd fields and even fields are displayed consecutively, e.g., a rate of 60 fields per second, for purposes of reconstructing an entire image at 30 frames per second (although various other rates may apply). Even though only half of the image is displayed per field, the rapid sequence of alternating odd and even horizontal scan lines creates the illusion of a full video image.
In the progressive format, a video image is represented entirely in a single frame that includes pixels on all horizontal lines of the full image. When displayed on a progressive-based display device, such as a computer monitor or of High Definition Television (HDTV), video images are presented in the form of progressively scanned frames, e.g., at a rate of 60 frames per second (although various other rates may apply).
Most conventional televisions support only the interlaced format and cannot perform progressive scanning. Accordingly, most video applications associated with display on televisions, including digital video players, such as DVD players, etc. are currently formatted in the interlaced format. As more progressive scan devices are adapted to play video on computers and HDTVs, however, there is a need for such devices to convert video from the interlaced format to progressive format.
The process of converting a video from the interlaced format to the progressive format is generally referred to as deinterlacing (also referred to as line-doubling). There are several ways to perform deinterlacing. Inexpensive devices may use what is known in the industry as the “weave” technique for performing the conversion from the interlaced format to the progressive format. Weaving simply merges even and odd fields together to make one progressive frame. If the video image is still (i.e., there are few or no moving objects in the image and/or objects that are moving are moving slowly), weaving tends to produce an image that is visually precise. However, if an image contains moving objects, particularly fast moving objects, the image produced on the display device tends to contain visual artifacts, such as visual distortions, feathering around the periphery of a moving object, flickering, pixel discontinuities, image inconsistencies, varying vertical resolutions, and other visual distortions. These artifacts tend to make images unsuitable for viewing on a HDTV or other high quality video players. These visual distortions are caused because there may be a time difference between when each odd and even field was recorded, and when they are combined the time differences produce the artifacts.
Another technique used to convert video from interlaced format to the progressive format is known as interpolation (also referred to the “Bob” technique). Interpolation involves converting each independent field into a complete single frame rather than combining even and odd fields to form a frame. But each field in the interlaced format only contains half the even or odd lines of a full resolution picture in the progressive format. To create a full frame, interpolation techniques involve filling-in so-called “missing lines,” meaning the even or odd lines that are not a part of the current field. For instance, if there is an even field, interpolation techniques will fill-in the missing lines, i.e., the odd lines, based on the pixel data from the existing, even lines to predict suitable pixel values for the pixels in the lines that are being interpolated, i.e., the missing pixels in the missing lines, and create a progressive frame. Correspondingly, if there is an odd field, interpolation techniques will fill-in the missing lines, i.e., the even lines, based on the existing odd lines to create a progressive frame. More specifically, interpolation techniques will fill-in the even or odd missing lines, depending on the field, by using pixels above and below those missing lines that are to be approximated through mathematical estimations. Unfortunately, these mathematical estimations are educated guesses that often produce artifacts, usually associated with motion and shifts of information in fields. In the world of HDTV, these artifacts are often sufficiently visually noticeable, making the visual experience on such devices undesirable.
For example, many of the techniques reconstruct a missing line by interpolating using the pixels above and below the missing line. For instance, an interpolated pixel, i.e., a pixel in an interpolated line, is generated using an average of the field pixel positioned immediately above and the actual field pixel positioned immediately below the interpolated pixel. Generally, the new pixel lines, or interpolated lines, are composed of pixels with an image consistency equal to an average value of pixel samples. This technique works well if the interpolated pixel is part of a video image feature forming a line (also referred to as an edge) of an image, which is either vertical, such as a telephone pole, or horizontal, such as the top of a wall.
However, if the line of an image is on a diagonal, then the aforementioned interpolation technique likely will produce artifacts, especially if there is motion associated with the image. As used herein, artifacts generally refer to visual distortions introduced into the image. For example, suppose the image involves a recording of a hockey game and the diagonal line is part of a hockey stick held at a diagonal angle, as is common. If the aforementioned interpolation techniques were used for deinterlacing an interlaced video of the hockey game, the hockey stick may appear to be blurred or jagged when the resulting progressive format is viewed.
Attempts have been made to improve these interpolation techniques by determining the slope, also referred to as a direction, for a diagonal line of a video image when converting from the interlaced format to the progressive format. Unfortunately, these techniques have a tendency to incorrectly determine the direction for displaying the diagonal line. Consequently, the resulting video image when viewed in a progressive format often includes undesired artifacts associated with the diagonal line.
There are other techniques that borrow from weaving and interpolation techniques, including combing the two using motion detection. Most, if not all these techniques utilize expensive equipment and tend to produce images that have visual artifacts that are noticeable to users. Thus, prior art techniques for minimizing visual artifacts when converting a video stream from an interlaced format to a progressive format have been unable to completely eliminate the visual artifacts caused by deinterlacing.